Re: [Qemu-devel] [PATCH for-2.7 00/15] block: Lock images when opening

[Fam Zheng <famz@redhat.com>]

On Wed, 04/13 13:18, Denis V. Lunev wrote:
> On 04/13/2016 12:09 PM, Fam Zheng wrote:
> >Too many troubles have been caused by two processes writing to the same image
> >unexpectedly. This series introduces automatical image locking into QEMU to
> >avoid such tragedy. With this, the user won't be able to open the image from
> >two processes (e.g. using qemu-img when the image is attached to the guest).
> >
> >Underneath is the fcntl syscall that locks the local file, similar to what is
> >already used in libvirt virtlockd.  Also because of that, we cannot directly
> >apply fcntl lock on the image file itself, instead we open and lock
> >"/var/tmp/.qemu-$sha1.lock", where $sha1 is derived from the image's full path
> >as in realpath(3). This mechanism should be equally useful for the single host
> >case, and it doesn't conflict with virtlockd when managed by libvirt.
> >
> >The alternative file locking API on Linux, flock(2), cannot protect host NFS
> >mount points, so it's not used.
> >
> >Gluster locking is also implemented wrapping glfs_posix_lock in patch 6. It's
> >only lightly tested.
> >
> >All other drivers that don't implement .bdrv_lockf are always permissive and
> >does no checking.
> >
> >In the future, the intention is that image format drivers that introduce
> >locking mechanisms could also fit into this API.
> >
> >The first 6 patches define the internal and external interfaces, and implement
> >the image locking.
> >
> >Patch 7 adds an option in qemu-io to allow disabling the lock, for testing
> >purpose.
> >
> >Patches 8 - 14 fixes the potential failures of test cases where multiple
> >processes may open the image concurrently.
> >
> >Finally the default behavior is switched to on in patch 15.
> >
> >Fam Zheng (15):
> >   block: Add BDRV_O_NO_LOCK
> >   qapi: Add lock-image in blockdev-add options
> >   blockdev: Add and parse "lock-image" option for block devices
> >   block: Introduce image file locking interface
> >   raw-posix: Implement .bdrv_lockf
> >   gluster: Implement .bdrv_lockf
> >   qemu-io: Add "-L" option for BDRV_O_NO_LOCK
> >   qemu-iotests: 140: Disable image lock for qemu-io access
> >   qemu-iotests: 046: Move version detection out from verify_io
> >   qemu-iotests: Fix lock-image for shared disk in test case 091
> >   qemu-iotests: Disable image lock when checking test image
> >   qemu-iotests: 051: Disable image lock in the command line
> >   ahci-test: Specify "lock-image=off" in CLI
> >   ide-test: Specify "lock-image=off" in command lines
> >   block: Turn on image locking by default
> >
> >  block.c                       | 25 +++++++++++
> >  block/gluster.c               | 34 +++++++++++++++
> >  block/raw-posix.c             | 97 +++++++++++++++++++++++++++++++++++++++++++
> >  blockdev.c                    |  8 ++++
> >  include/block/block.h         |  9 ++++
> >  include/block/block_int.h     |  5 +++
> >  qapi/block-core.json          |  6 ++-
> >  qemu-io.c                     | 22 +++++++++-
> >  tests/ahci-test.c             | 16 +++++--
> >  tests/ide-test.c              |  5 ++-
> >  tests/qemu-iotests/030        |  2 +-
> >  tests/qemu-iotests/046        | 22 +++++-----
> >  tests/qemu-iotests/051        |  2 +-
> >  tests/qemu-iotests/051.out    | 10 ++---
> >  tests/qemu-iotests/051.pc.out | 10 ++---
> >  tests/qemu-iotests/091        |  4 +-
> >  tests/qemu-iotests/140        |  2 +-
> >  17 files changed, 245 insertions(+), 34 deletions(-)
> >
> First of all, I like the approach as we have discussed :) Then,
> in general, I support Daniel with the point that
> the locking should be done on the image file
> directly.
> 
> Also, it looks like this will break migration with the shared storage.
> For me it seems that we will have lock the image from both ends

There should already be state handling logic between source and destinition
ends, I'll take a look and see if we can add lock/unlock calls there to keep it
working.

Fam